This project investigates the role of lateral molecular motion of cell surface components, particularly the acetylcholine receptor, in the formation of nerve/muscle synapses. Fast lateral motions are measured by the fluorescence photobleaching recovery technique; slower changes in distribution are viewed by time-lapse video recording of fluorescence labeled muscle cells and neurites. The biological systems under study include cultures of rat primary myotubes, intact single muscle fibers, and embryonic spinal cord explants. The causes and consequences of acetylcholine receptor aggregations on muscle induced by means other than neuronal contact are also studied.